1. Field of the Invention
The present invention relates to a process for the production of a solid solution carbide of (Mo, W)C or a carbonitride of (Mo, W)(CN), which are used as a raw material for cemented carbide alloys, and more particularly, it is concerned with a process for the production of such a carbide or carbonitride with a uniform particle size.
2. Description of the Prior Art
Up to the present time, as a starting material for cemented carbides, there has been used tungsten carbide (WC) as a major component, but tungsten is found in only a few parts of the world and thus is very expensive. Lately, the tendency is to replace WC molybdenum carbide (MoC) having the same crystal structure as WC as well as mechanical properties similar to WC and since MoC is unstable, MoC is stabilized by dissolving WC therein to form a solid solution of (Mo, W)C which is is used as a starting material for cemented carbide alloys.
When using such a carbide or carbonitride as a raw material for cemented carbides alloys or hard alloys, it is most important how to control the particle size of the hard phase in the alloys and the thickness of the binder phase corresponding thereto and to this end, the particle size and eveness of the raw material powder such as (Mo, W)C or (Mo, W)(CN) powder are most important for making even the particle size and distribution there of the hard phase.
In the production of mixed carbides of Group IVa, Va and VIa metals of Periodic Table, metal oxides, carbides and carbon are mixed correspondingly to the composition of an object compound and reacted at a high temperature, or the reaction is promoted by adding an additive to increase the diffusion rate. When a solid solution is produced by the solid phase reaction of powders, however, the degree of reaction is scattered depending on the mode of mixing the powders, the particle size and size distribution of the powders used. In order to form a uniform solid solution, a heating operation for a long period of time is necessary as in the process described in Japanese Patent Application (OPI) No. 146306/1976 in which a part of Mo in MoC is replaced by W to stabilize the (Mo, W)C phase of simple hexagonal type. That is, in the production of a uniform solid solution by the diffusion among powders of metals such as Mo and W and carbides, heating at a high temperature such as 1600.degree. C. or higher for a long time is required, in particular, for diffusing and dissolving metallic powders of Mo and W with a particle size of several microns.
As a result of examining the particle size and the particle size distribution of (Mo, W)C and (Mo, W)(CN) prepared by such a known method, a reaction mechanism is found as shown in FIG. 1. In the method as shown in Japanese Patent Application (OPI) No. 146306/1976 and Japanese Patent Application (OPI) No. 104617/1978 wherein predetermined amounts of MoC and WC to give a final carbide (Mo, W)C are previously mixed, large amounts of carbon and an iron group metal such as Co or Ni for stabilizing (Mo, w)C are added before the reaction (a-1). During the course of the reaction, there appears once a stable form of (Mo, W).sub.2 C+C(a-2), but when this is converted into (Mo, W)C by a subsequent heat treatment, the particle size or diameter fluctuates (a-3).
If the powder particles are very fine, on the other hand, diffusion proceeds well and a large amount of an iron group metal as a diffusion aid is not required, resulting in a good quality carbide. However, it is difficult on a commercial scale to obtain powders of metals and carbides with a particle size of 0.5 micron or less.
We, the inventors, have hitherto found that when Mo and W are mixed in the form of ammonium salts of Mo and W, in the state of their solutions or in the form of their oxides or halides, mixing can better be accomplished and a uniform solid solution can more readily be obtained at a relatively low temperature as compared with combinations of metal powders and/or carbide powders. In this case, for example, W and Mo are uniformly mixed at the stage of forming their oxides and reduced with hydrogen to form a solid solution of (Mo, W) which is then reacted with carbon to give a solid solution carbide. This has already been proposed as a commercially feasible process (U.S. Pat. No. 4,216,009).